Again About DBB, DIB and DPE 1

[JohnLL]

I have read almost all of the topics in Eng-tips about ball valves and Double block and bleed (DBB), Double Isolation and Bleed (DIB), etc.
I also read the API 6D definitions and tests (DIB-1 and DIB-2).
BUT STILL STUCK THERE!

I would appreciate if somebody can answer my question, put in simple word as follows:

I need a ball valve, which could be checked for leakage before doing maintenance on downstream. So, I need blocking and also bleed on cavity.
I need the valve be bidirectional.
finally, I need the valve use self-relieving seats (no cavity relief valve used).

Therefore, I think one seat SPE and one seat DPE is not an option because it will make the valve suitable for one-direction.
and both seats DPE is not an option too, because I need self-relieving.
Now is DBB the answer? If a valve is DBB, then could it have self-relieving seats?
does it have a good seal while during operation the pressure is just on one side (upstream of course)?
and if I use that, then in case of pressure increase in cavity, it always drain to downstream?

Please advise!

 

[zdas04]

One word answer? As was said to Dustin Hoffman in The Graduate "Plastics".

If I can use more than one word, you requirements are EXACTLY the capabilities of a Trunnion Ball Valve. One to One match, I've never seen a better fit.

David

 

[superweo]

Dear JohnLL
Trunnion mounted ball valves are always DBB, no matter if self relieving or double piston.
Usually if you choose self relieving seats the overpressure is discharged in both sides of the valve.

Ciao

 

[HitokiriAoshi]

don't let DBB DBI confuse you. Here is the simplest way to procure a ball valve.

Upstream seat.
[X] seal against upstream pressure
[ ] seal against cavity pressure (non relieving seat)

Downstream seat
[X] seal against downstream pressure
[ ] seal against cavity pressure (non relieving seat)

Cavity
[ ] Bleed hole (choose if DBB)
[ ] Self relieving valve installed (required if both seats seal against cavity)

now for your problem, you need to know what "bi-direction" you need. are you going to have pressure sometime at both sides of the valve? both DPE and SPE seats will seal against pipeline pressure, the difference is how they deal with cavity pressure.

-> SPE CAVITY DPE ->
any overpressure of cavity will leak back upstream a "true isolation"
<- SPE Cavity DPE <-
Any overpressure of cavity will leak "downstream", your DBI valve in reverse now is just a DBB valve